I have developed a novel micromachined GaAlAs/air mirror technology for optoelectronic applications.
More particularly, I have developed a series of advanced micro-mechanical optoelectronic devices based on a novel broad-band multilayer GaAlAs/air mirror technology (see FIGS. 1A-1D). My new micro-mirror technology is fabricated by epitaxial growth of GaAs/GaAlAs structures, followed by highly selective lateral etching of the high-aluminum-content GaAlAs layers. Because of the large index difference between the GaAs and air layers (3.5 and 1), the resulting multilayer GaAlAs/air structure is an extremely efficient multilayer mirror, with very broad bandwidth. It can be shown that with only three (3) periods of my mirror structure, reflectivity of over 99.990% can be achieved with 700 nm (nanometer) bandwidth. By comparison, over twenty (20) periods of standard GaAs/AlAs structure are needed to achieve 99.900% reflectivity, at the cost of limited bandwidth of less than 25 nm.
My simple but powerful micromachined structure can be applied to solve a variety of technological problems, and allows the fabrication of new devices where broadband mirrors are required.
A short list of technological breakthroughs resulting from this concept are as follows:
Broadly Tunable Fabry-Perot Filters
A novel tunable Fabry-Perot filter based on my GaAlAs/air mirror is shown in FIG. 1B. The device consists of top and bottom GaAlAs/air mirrors and an AlAs cavity spacing which is etched away to allow formation of a cantilever. Application of an applied field will change the cavity length and shift the transmission peak of the Fabry-Perot filter. The broad (700 nm) bandwidth of the mirror allows a tuning range of over 500 nm. The high reflectivity and high quality of the mirrors will allow better than 1 angstrom linewidth.
However, due to the bending of the cantilever, the Fabry-Perot interfaces shown in FIG. 1B will not remain parallel, causing a slight broadening of the linewidth. For many applicationsxe2x80x94such as switchingxe2x80x94this will not be important. For other applications where the narrow linewidth is critical, a xe2x80x9ctrampolinexe2x80x9d structure has been designed and is presented.
The device can further be integrated with laterally grown detectors for a wide range of spectroscopic applications such as environmental monitoring (e.g., toxic gases such as methane or acetylene, with absorption lines at 1330 nm and 1770 nm), or biomedical applications such as the measuring of blood sugar levels requiring spectroscopy near 2100 nm, etc.
Furthermore, the small size, and the compatibility of the devices with multimode fibers, is very attractive for such commercial applications.
Fixed Wavelength VCSEL""s
A critical parameter in fabricating vertical cavity surface emitting laser (VCSEL) devices is that the top and bottom mirror reflectance xe2x80x9cpeaksxe2x80x9d, and the laser xe2x80x9cexciton peakxe2x80x9d, must correspond. Considering the narrow spectral response (25 nm) of GaAs/AlAs, and the inhomogeneous growth of GaAlAs across a wafer (more than 3% composition variations resulting in over 30 nm variation), makes the growth of such devices low yield and costly. My GaAlAs/air mirror technology shown in FIG. 1C will impact this technology in two ways:
(i) The broad bandwidth of the GaAlAs/air mirrors will relax the restrictive growth conditions since the mirrors are broadband, allowing perfect overlap with the exciton peak throughout a wafer. This will allow fabrication of working VCSEL devices with GaAlAs/air mirrors from any part of the wafer.
(ii) The higher mirror reflectivities will reduce the laser threshold conditions by increasing the Q-factor of the laser cavity.
Tunable VCESL""s
The new tunable filter technology shown in FIG. 1B, and the new VCSEL technology shown in FIG. 1C, can be combined to yield a new tunable VCSEL technology as shown in FIG. 1D. In this new tunable VCSEL, the top GaAlAs/air mirror is formed into a cantilever which can be moved up and down electrostatically to tune the lasing wavelength. Again, the high reflectivity and the wide bandwidth of the mirrors will allow the laser to emit continuously and over a wide wavelength range. The tuning range will, of course, be limited to the gain bandwidth of the diode, i.e., approximately 50 nm.
My new tunable filter device consists of a GaAs micromachined tunable filter chip and can be as small as 500 microns by 500 microns, with a response time of several microseconds. The cost of this device is low, since thousands of GaAs chips are mass-manufactured using conventional semiconductor processing techniques. One of the most important features of my new device is its improved spectral resolution and bandwidth, due to my approach for fabrication of very high quality mirrors with minimum complexity. As a result, the resolution and the bandwidth of this device is about an order of magnitude better than other micromachining-based technologies.
FIG. 2 illustrates the evolution of tunable filters over the last ten years. Current state-of-the-art filters are typically hybrid, making it difficult to fabricate them in large volumes. They generally either rely on the use of piezoelectric drivers with complex feedback systems (Queensgate Instruments, England), or on the use of birefringent materials placed between crossed polarizers (for example, Cambridge Research Instruments, Massachusetts, using liquid crystals).
Piezoelectric tunable filters generally have a resolution of 0.1 nm, with 50 nm tuning range (bandwidth).
Liquid crystal (LC)-based filters can exhibit better resolutions, but at the expense of very low efficiency, e.g., as low as 99.0%.
The fabrication of these hybrid systems is a labor-intensive process, thus increasing the cost of these devices. For example, the top-of-the-line model sold by Queensgate Instruments costs above $10,000. Such high costs make them unrealistic for most applications. Specifically, in upcoming communications networks, it is anticipated that all of the information delivered to each household (500 channel TV, telephone, etc.) will be transmitted over a fiber optic line using wavelength division multiplexing. This will necessitate the use of a tunable filter in every home. In order for such a system to be feasible, the cost of a tunable filter should be in the tens-of-dollars range, at the most.
In attempting to adopt micromachined technologies for fabrication of low cost tunable filters1,2,3, a number of approaches have been initiated.
Larson and et al. built a GaAs-based interferometer1. They used a GaAs/AlAs stack as the bottom mirror and a gold-coated silicon nitride membrane as the top mirror.
Jerman and et al.2 bonded two different wafers to build their micromachined membranes. They used dielectric mirrors with 97.5% refractivity at 1.55 mm2.
In the work of Reference 3, a silicon nitride membrane is suspended over a silicon substrate. The device is used as a light modulator based on the interference effect between the substrate and the suspended membrane.
All prior micromachined filter technology tends to suffer from the limited reflectivity and bandwidth of the cavity mirrors.
The bandwidth xcex94xcex of a periodic layered structure (with indices n1 and n2) is given by the well know formula4:
xcex94xcex/xcex=(4/xcfx80)sinxe2x88x921((n2xe2x88x92n1)/(n2+n1))
This shows a direct link between the index difference between layers and the bandwidth.
Similarly, the peak reflectivity is a function of the number of layers in the dielectric stack and the index difference between the layers.
When used in a tunable Fabry-Perot device (e.g., filter or VCSEL), both the reflectivity and the bandwidth of the mirrors play key roles: the reflectivity determines the spectral resolutions of the Fabry-Perot device and the bandwidth limits the tunability range. Therefore, it is desirable to have as large of an index difference as possible in order to achieve highly reflective and broadband mirrors.
As mentioned above, a conventional mirror stack consists of GaAs/AlAs layers with closely matched indexes of refraction (3.5 vs. 3.0). It is therefore difficult to make high quality Fabry-Perot structures using these layers.
I substitute the low index AlAs material with the following:
(i) Air Gaps: By selectively etching AlAs (or high aluminum content GaAlAs) layers (using HF or HCl based solutions) from the original GaAs/AlAs stack, one can achieve a mirror stack consisting of GaAs/air gaps. As air has an index of refraction of 1, this results in the highest possible index difference using GaAs technology. The selective etching region can also be GaAs, in which case it can be removed by conventional citric acid solutions.
(ii) Oxidized AlAs: AlAs (or high aluminum content GaAlAs) layers in epitaxially grown GaAlAs materials can be oxidized laterally using conventional wet oxidation techniques. Oxidized AlAs has an index of refraction of 1.5, as opposed to 3.0 of non-oxidized AlAs. This is a significant index difference, and mirrors made of oxidized AlAs/GaAs stacks approach the quality of GaAs/air gap stacks in terms of bandwidth and reflectivity.
FIG. 3 illustrates the number of layers in a dielectric stack required to achieve a reflectivity approaching 100%. In the case of GaAs/AlAs, 15 layers are needed to obtain 99% reflectivity, whereas with GaAs/air mirrors, only 4 layers are needed to achieve 99.999% reflectivity.
Although oxidized AlAs/GaAs based mirrors are inferior to GaAs/air gap mirrors, they may have a higher fabrication yield due to the inherent mechanical stability. However, as shown below, their fabrication as a whole does require more photolithographic steps.
The use of these low index air gap or oxidized AlAs layers increases the bandwidth of my devices by an order of magnitude compared to other known devices. Similarly, my devices can achieve a resolution in the 0.1-0.3 nm range. This is comparable to the best filters available with the expensive hybrid technology, and one order of magnitude better than with other micromachining-based approaches.